This invention relates to an image forming apparatus which uses a light receiving sheet coated with microcapsules and more particularly to such an image forming apparatus provided with a supplementary exposure device for correcting the radiationcurability characteristics of the light receiving sheet.
Japanese Patent Publication No. Tokkai 58-88739 has disclosed a method for forming a visible image by exposing a light receiving sheet: coated with photosensitive microcapsules encapsulating within a resin shell a colorless dye and a radiation-curable material to an image forming beam of light reflected from a document to be copied to thereby form a selectively hardened image, superposing on this selectively hardened image an image receiving sheet coated with a developing agent having a chromogenic effect on the colorless dye and pressing these two sheets together. If use is made of colorless dyes for coloring in yellow, magenta and cyanic encapsulated together with radiation-curable materials sensitive to blue, green and red, respectively, and if these microcapsules encapsulating these three different combinations of a colorless dye and a radiation-curable material are uniformly distributed over the surface of a light receiving sheet, a full color image can be produced by exposing such a sheet only once to an image forming white beam of light carrying color data. This method of forming a full color image is advantageous over the conventional electrophotographic method because the image forming process can be significantly simplified.
Light receiving sheets of this type have their own peculiar radiation-curability characteristics and the quality of the image formed thereon depends significantly on these characteristics. For example, if the so-called gamma value of the light receiving sheet is too large, gradation becomes low and it becomes difficult to reproduce intermediate colors. In the case of a photograph, for example, the image quality becomes poor without the intermediate colors faithfully reproduced. In Japanese Patent Application No. 62-198746, the present inventors disclosed an image forming apparatus provided with a supplementary exposure device for correcting the radiation-curability characteristics of the light receiving sheet such as the gamma. With a supplementary exposure of the light receiving sheet, the oxygen content inside the photosensitive microcapsules can be reduced such that the radiationcurability characteristics of the light receiving sheet are corrected and images with high gradation values can be formed.
Although such a supplementary exposure device makes it possible to obtain high-quality images with high gradiation values in the case of photographs with intermediate colors, a high gradiation level makes an image of low quality in the case of characters and graphs. In the case of a typewritten document, for example, the ink density is not always uniform and if such a document is copied by using a copier having a supplementary exposure device of the type described above, the non-uniformity of ink density is reproduced as on the original document. This makes the copied characters unclear and difficult to read.